Create Selection API

[zachcp]

• core/selection
• looks like this:

  let ac = AtomCollection::from(&pdb);
  assert_eq!(ac.size(), 1413);

  let selected_atoms = ac
      .select()
      .chain("A")
      .residue("GLY")
      .element(Element::C)
      .collect();

assert_eq!(selected_atoms.size(), 22);